Resistance welding, or spot welding, is used to join metal members, such as sheets or structures. The members to be welded are typically held together between a pair of electrodes. As the electrodes contact outer surfaces of the members, electric current passes through the members between the electrodes. The resistance of the members creates heat, which melts a portion of the members between the electrodes to form a spot weld.
When a spot weld gun is used in robotic applications, the electrodes are commonly supported by pivoting arms actuated with a pneumatic cylinder or a servo actuator. Because of mounting restraints and inertia the electrodes of the weld gun do not actuate evenly, causing the electrodes to contact one of the metal members before the other. Equalizing air cylinders and springs have been used to synchronize the motion of electrodes however, they have been unsuccessful because of inertia and the inability to properly coordinate the actuator and the equalizers.
When the electrodes contact members off centered or with unequal force, the electrodes can bend the assembly. A method of moving the electrodes in a predetermined manner so that they contact the members simultaneously at a predetermined point is desired.